Add validation in remove blobs feature
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We currently have a feature to [remove blobs](https://docs.gitlab.com/user/project/repository/repository_size/#remove-blobs) from a Git repository. However, there are some conditions that will prevent these blobs from being removed, such as if the repository is part of an object pool. We should build in validation when the user tries to remove the blob and produce an error message if we know we cannot remove the blob on the backend.
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